Add release detail to CONTRIBUTING.md

[justinwb]

At the moment we have a stable v4 release represented in master, and a beta release in it's own branch (release/v5.0.0). @csarven made a good point (see https://gitter.im/solid/node-solid-server?at=5c4c5a589221b9382dc52a3f) that it isn't clear for people wanting to contribute to one or the other where they should fork from / pull to if they want to contribute. I suggest adding this detail to CONTRIBUTING.md, at least while there are parallel version tracks going on.